Mystery of Honeycomb Cloud Formation Solved

Easily spotted by their honeycomb shape, open-cell clouds are one of the most common cloud formations, found on the backside of low pressure systems and skirting the edges of every continent. Yet for all their ubiquity, they are among the more mysterious cloud formations known, and rules guiding the formation of open-cell clouds have not been quantified — until now.

Starting with a computer model of cloud formation developed at the National Center for Atmospheric Research, climate physicists refined and reconfigured its internal dynamics until they matched patterns seen in the real world. The math is complicated in detail, but simple in principle.

“Imagine you had a hosepipe, pointed it at the ground, and turned it on. The water rushes out hard, hits the ground and is forced to diverge. Now imagine you’ve got not just one hosepipe, but many. All these diverging flows start hitting each other. The water has to go somewhere, and the only place it can go is upwards,” said Graham Feingold, a National Oceanic and Atmospheric Administration physicist.

To fit the analogy to open-cell cloud formation, replace the water with air that’s pulled downwards by raindrops that cool as they evaporate. The jets of air hit the ocean, split into streams flowing across the water, and collide with other jets, driving them back up into the atmosphere. Once there, water droplets form around tiny particles of dust and biological debris, eventually coalescing into clouds. Their pattern is dictated by geometries of airstream collision far below.

Feingold’s team tested their simulation against a month of real-world cloud data gathered from a site off the coast of South America by satellites, aircraft, boats, underwater and on-the-water sensors, under varying wind strengths, humidity levels and precipitation rates. At every level, the researchers’ simulations matched what was observed. “We bring all these measurements to bear to look for the same signatures that we find in the models,” said NOAA physicist Alan Brewer.

The findings, published August 12 in Nature, should help climate modelers improve the fine-grained dynamics of coarse-resolution models. Clouds are an especially important component, since their formation affects how much sunlight is reflected and how much hits Earth directly. Depending on the relationship between regional climates and global warming, clouds could slow rapid warming — or let it run away.

With the basic equations of open-cell cloud formation in hand, the researchers next want to study how they become “closed” cells, an inverse pattern in which the honeycomb shape is produced by dense clouds outlined by narrow gaps. “We’ve seen how it transitions from closed to open, but we’re very unclear on how it could move back to closed,” said Feingold.

The patterns of open-cell cloud formation are typical of self-organizing systems, said the researchers. Similar patterns are seen in bird flocks, crystal growth, social networks and many ecosystems. Why such different systems should display common behaviors is still being explored.

“We’ve also discovered that oscillations start to occur. Precipitation is synchronized. We’re very fascinated by this,” said Feingold. “The realm of self-organizing properties is wide open, and we’ll pursue it.”

More cloud images on the following page.

Images: 1) Alternating bands of closed-cell (light) and open-cell (dark) cloud formations off the coast of Peru./NASA. 2) Simulations of closed-cell cloud becoming open-cell./Nature. 3) Open-cell cloud formation over the Bahamas./NASA. 4) Over South Georgia Island./NASA 5) Off the west coast of South America./NASA.